Brian Gardner

Integrated Land Use-Transport Model System with Dynamic Time-Dependent Activity-Travel Microsimulation

The development of integrated land use–transport model systems has long been of interest because of the complex interrelationships between land use, transport demand, and network supply. This paper describes the design and prototype implementation of an integrated model system that involves the microsimulation of location choices in the land use domain, activity–travel choices in the travel demand domain, and individual vehicles on networks in the network supply modeling domain. Although many previous applications of integrated transport demand–supply models have relied on a sequential coupling of the models, the system presented in this paper involves a dynamic integration of the activity–travel demand model and the dynamic traffic assignment and simulation model with appropriate feedback to the land use model system. The system has been fully implemented, and initial results of model system runs in a case study test application suggest that the proposed model design provides a robust behavioral framework for simulation of human activity–travel behavior in space, time, and networks. The paper provides a detailed description of the design, together with results from initial test runs.

INCORPORATING FEEDBACK IN TRAVEL FORECASTING

The Clean Air Act Amendments of 1990 (CAAA) introduced new requirements for how transportation modeling for air quality analysis must be performed in nonattainment areas. Because of the degree to which vehicle emission rates (on a grams-per-mile basis) are affected by speed, specific attention has been given to how speeds are estimated and subsequently used in the travel forecasting and emissions estimation process. CAAA and guidelines issued in the years following introduction of the act require that speeds used in the process be realistic in comparison to what might be observed on the road and be reasonably consistent throughout the modeling process. In most traditional modeling processes that model trip generation, trip distribution, mode choice, route assignment, and emissions separately and sequentially, it has not been unusual to find different speeds (and travel times) used in different parts of the process. A description of two different research and development efforts that have produced new methods and guidelines for introducing feedback into the travel and emissions forecasting process to ensure consistent use of speeds is provided. COMSIS Corporation developed for FHWA a method for introducing feedback into the traditional four-step process by using an iterative process through all of the steps until the process converged to a stable set of link speeds. The methodology was used to test the effects of introducing feedback on model results under different levels of network congestion (feedback affects the results only when there is congestion in the network). The project resulted in a report documenting the methods, pitfalls, and common concerns for introducing feedback. A summary of the research conclusions from the project is provided.

ESTIMATING COSTS AND BENEFITS OF TRANSPORTATION CORRIDOR ALTERNATIVES

Although benefit-cost assessment is a useful tool in structuring the decision making process, it has not generally been used to assist in multi-modal decision making in metropolitan areas. Also, although detailed zone-to-zone trip information can be obtained from metropolitan travel-demand models, this information is not currently used by planners in developing detailed information on cross-modal comparisons of costs and benefits. A real-world application of benefit-cost analysis for multi-modal decision making using detailed zone-to-zone trip data output from travel-demand models for the I-15 corridor in Salt Lake City is presented. The analysis was conducted at two levels: corridor and region-wide. The research suggests that, when major investments are to be evaluated, the analyst should be very cautious in performing corridor-level analyses when such a trip-based approach is used, because of significant effects on the evaluation caused by traffic diverted into (or out of) the corridor.

Evaluation of Large-Truck Transportation Alternatives with Safety, Mobility, Energy, and Emissions Analysis:

During 2008, more than 5.8 million motor vehicle crashes, resulting in 37,000 fatalities and 2.3 million injuries, were reported to police in the United States. Research has identified heavy trucks as significant contributors to unsafe traveling conditions because of the different operating and maneuvering capabilities of trucks. Three safety treatments—peak hour travel restrictions, speed limiters, and lane restrictions—have been proposed to alleviate the safety risks of heavy trucks in the traffic stream. However, it is often difficult to quantify the potential safety benefits of transportation alternatives because crash occurrences, the ultimate indicators of the safety of a roadway, are sporadic, random events. A safety evaluation module was developed for the TRANSIMS microsimulator to simulate the safety of a transportation network with respect to rear-end crashes with safe headway distance used as a crash surrogate. The safety evaluation module could measure the safety benefits of competing transportation alternatives. Simulation results indicated that left-lane restrictions were the most statistically significant and beneficial treatment strategy; network-aggregated safe headway distance as an indicator of the likelihood of a rear-end crash decreased from pretreatment levels by at least 2% and 1% for the off-peak and peak hour travel periods, respectively. Supplementary analysis with the microscopic energy and emissions model from Virginia Polytechnic Institute and State University indicated that left-lane restrictions also had marginally positive effects on fuel consumption and emissions.

Evaluating the Options in Urban Areas with Financial Constraints

Logic is devised for a needs test for new general-purpose lane (GPL) highway capacity in urban areas that have limited funding available for new infrastructure investments. GPL capacity is defined as mixed-flow lanes on which both single-occupant and high-occupancy vehicles are permitted. Methodologies to apply the needs test and to evaluate the options in urban areas facing limitations on new GPL capacity are developed. A case study is used to demonstrate the methodologies to evaluate the air quality and cost-effectiveness impacts of transportation system alternatives, illustrating how planners may develop the type of information that policy makers will need to help them make informed decisions about long-term options.